Platinum-modified aluminide (PtAl) coatings have been formed by electroplating Pt onto a substrate, applying a short high temperature heat treatment, applying a layer of Al, and performing a diffusion heat treatment cycle.
Using this basic process, it is possible to make different classes of PtAl coatings. The resulting phase structure of the resulting coatings can depend upon a number of factors, such as, for example, the thickness of the initial Pt plate, the post-plate diffusion cycle parameters, and the type of aluminizing cycle.
When applied after the short high temperature heat treatment, a low temperature high activity aluminizing cycle process (LTHA) typically yields a two-phase PtAl coating with an outer layer making up about 65% of the total thickness, with the diffusion zone accounting for only about a third of the coating thickness. LTHA processes also typically require a post-coat diffusion heat treatment to form the final coating. For such two-phase coatings, the Pt content in the outer layer can range from about 25 wt. % to about 50 wt. % or higher. If a high temperature low activity (HTLA) aluminizing process is applied following heat treatment, the resulting PtAl coating may be either two-phase or single-phase, depending upon the specific process parameters. The HTLA process usually includes adequately high temperatures to allow the aluminum to diffuse into the PtAl structure during the application step. The single-phase PtAl coatings formed by a HTLA process generally have Pt contents in the 15-30 wt. % range, with the outer zone of the coating making up only approximately 50% of the total thickness. The diffusion zone is generally from about 40% to about 50% of the total thickness.